An area of low pressure will approach the Mid-Atlantic on Thursday afternoon, passing through the DC Area on Friday increasing the risk of strong to severe thunderstorms and very heavy rainfall.
The image below is of the latest Global Ensemble Forecast System model, showing decent agreement that a surface low pressure system will track out of the Ohio Valley Thursday night and pass very near or right across the DC Region on Friday into Saturday morning.
Precipitable Water (The amount of water vapor, in a column of air, that is equivalent to the depth of the liquid water that would be obtained if it fell as rain) is forecast to rapidly increase on Friday when widespread showers and embedded thunderstorms could put down torrential rainfall across our region.
Strong to Severe Thunderstorms Possible Thursday Afternoon:
The latest high resolution North American Model (NAM) shows strong thunderstorm breaking out on Thursday afternoon beginning in the Shenandoah Valley and then moving east across the Washington Region.
Threat Shifts to Torrential Rain on Friday:
On Friday as the surface low and upper level energy approaches from the west, deep-moist convection and strong surface convergence should allow for squall-lines containing very heavy rainfall to push across our area in multiple waves.
Training thunderstorms (storms that repeatedly track over the same locations) could cause creeks and streams to flow out of their banks and urban flooding is possible in low-lying, poor drainage areas. If the latest forecast models are accurate, Friday’s rush hour may be greatly impacted.
Latest indications are that this system should move off of our coast on Saturday morning, with clearing conditions Saturday afternoon and much lower humidity through Sunday with sunshine.